Field of the Invention
The present invention relates to a fuel cell system.
Background Art
There has been known a fuel cell system applied to a fuel cell automobile or the like. The fuel cell system is a system that mainly includes a fuel cell stack, as its main unit, provided with an anode electrode to which hydrogen is supplied as a fuel gas and a cathode electrode to which air is supplied, a tank storing a fuel gas, such as hydrogen, to be supplied to the anode electrode, and a hydrogen circulation mechanism (hydrogen pump) for returning an exhaust gas which contains an unused fuel gas back to the anode. In the fuel cell stack, hydrogen and the oxygen contained in air react to generate power.
It is known that, as the reaction proceeds in the fuel cell, the nitrogen in a cathode gas (air) or the water generated by the reaction oozes out from the cathode electrode to the anode electrode through an electrolyte membrane. This increases the partial pressure of nitrogen and the like (hereinafter collectively referred to as “impurities”) at the anode electrode, and the concentration of the fuel gas (hydrogen) decreases, undesirably deteriorating the power generation performance of the fuel cell.
Generally, therefore, in a fuel cell system provided with the foregoing hydrogen circulation mechanism or the like, a discharge valve provided in a discharge flow path on the anode electrode side (hereinafter referred to also as “the hydrogen system”) is opened to discharge unused hydrogen or a gas containing impurities. For example, Patent Document 1 describes a system in which the concentration of the impurities in the hydrogen system is estimated on the basis of the pressure of the hydrogen system of the fuel cell and the hydrogen partial pressure in the hydrogen system, and a discharge valve is opened according to the estimated concentration of the impurities, thereby discharging a hydrogen off-gas (unused hydrogen or a gas containing impurities discharged from the anode electrode).